The design of vacuum glazing involves a complex set of trade offs between thermal performance and stress. In particular, the support pillars serve to concentrate the forces due to atmospheric pressure, leading to high levels of stress in the glass in the immediate vicinity of the support pillars. Such stresses can lead to local fractures of the glass. Further, the glass sheets bend over the support pillars giving rise to regions of tensile stress on the external surfaces of the glass sheets immediately above the support pillars. In addition, the pillars themselves experience high levels of stress, and must be made out of a material which has a very high compressive strength. Finally, the support pillars themselves act as thermal bridges between the glass sheets, leading to heat flow through the glazing.
Substantial progress has been made in the design and manufacture of vacuum glazing over the last few years. Vacuum glazings up to 1 m.times.1 m have been produced with high levels of thermal insulation. It has been shown that reasonable design compromises can be achieved between the competing constraints associated with mechanical tensile stress on the one hand, and heat flow through the glazing on the other.
The support pillars concentrate the forces due to atmospheric pressure leading to high stresses in the glass and in the pillars. The nature of this stress concentration is well understood. The probability of fracture due to the concentrated forces can be determined by reference to the literature on indentation fracture of glass. In the design approach for vacuum glazing, dimensions of the pillar array are chosen to ensure that the formation of classical conical indentation fractures in the glass due to the support pillars should not occur.
Experience with the production of vacuum glazing has shown that there is another mode of fracture which can occur in the glass sheets near the support pillars. These fractures arise because of shear (sideways) stresses between the glass sheets and the pillars. The shear stresses are associated with in-plane movement of one glass sheet relative to another. Such movement can occur because of bending of the glass sheets, particularly complex bending modes in which the sheets are not deformed spherically, or because of temperature non-uniformities in either glass sheet. Either influence tends to cause the interface between the pillar and one glass sheet to move sideways relative to this interface on the other sheet. The large axial force between the pillars and the glass sheets prevents the contacting surfaces from moving relative to each other. This results in shear force between the support pillar and the glass sheets and leads to small crescent shaped fractures in the glass sheets adjacent to the pillars. The fact that these fractures are associated with shear stress can be confirmed by observing that they tend to be seen in pairs, on opposite edges of the support pillars in either glass sheet.
One of the reasons why these shear stresses occur is because, in practical designs of vacuum glazing, the support pillars must be made of material of very high compressive strength. If the pillars are not of high enough compressive strength, they deform inelastically during the establishment of the vacuum in the glazing, leading to large bending of the glass sheets in the vicinity of the edge seal. The fact that the support pillars are of high strength means that they do not deform significantly when shear forces are present.